Tree Diseases: Bacterial Leaf Scorch (Xylella fastidiosa)
Bacterial leaf scorch is a disease of various shade trees, perennials, and crops that is caused by the bacteria Xylella fastidiosa. The bacteria invade the xylem of susceptible trees, and multiply. The xylem rapidly becomes clogged, disrupting the tree’s vascular system. As the infection advances, it causes the tree to experience significant foliar dieback. There is no remedy for the disease. As such, severe infections often culminate in plant mortality.
Distribution & Habitat
Bacterial leaf scorch occurs globally, wherever susceptible trees are present.
Bacterial leaf scorch infects a bevy of hosts, including almond, American beautyberry, American elm, blackberry, Boston ivy, English ivy, bur oak, pin oak, red oak, shingle oak, white oak, willow oak, red maple, sugar maple, mulberry, sweet gum, poison hemlock, peppervine, umbrella sedge, dallis grass, wild strawberry, grapes, miner’s lettuce, eastern baccharis, sumac, periwinkle, goldenrod, American elder, peach, and Virginia creeper.
The pathogen that causes bacterial leaf scorch is spread to susceptible plants via xylem-feeding insect vectors, particularly leafhoppers, sharpshooters, spittlebugs, and treehoppers. As the insects feed on the terminal shoots of vulnerable hosts, they transmit the bacteria into the plant’s xylem. While this is the most common means of transmission, the bacteria may also invade hosts through root grafts.
Once the bacteria have become established within a plant, they colonize the xylem vessels, and begin to multiply. As the bacteria increase in population, the xylem becomes congested. This disrupts the plant’s vascular system, inhibiting the transportion of water and nutrients throughout the crown. Eventually, the bacteria will infect other portions of the plant, resulting in a widespread scorching of diseased foliage. The bacteria continue to multiply within the xylem throughout the growing season. In late fall, as temperatures cool, the bacteria enter dormancy, and overwinter in the infected plant material. They resume activity the following spring.
Symptoms of Infection
Diseased plants start to exhibit symptoms of infection in mid-summer. Infected foliage will experience a premature browning. At first, this necrosis occurs along the leaf margins, before spreading inwards toward the leaf veins and petiole. As the infection deepens, yellow or reddish-brown bands form between the healthy and necrotic tissue. Once the leaves become necrotic, they may be cast from the tree.
Bacteria leaf scorch symptoms will appear on the same limbs each year. Over the next 3 to 8 years, the bacteria will gradually spread, until the entire crown has been infected. The lack of healthy growth on diseased plants will result in extensive foliar dieback, followed by plant mortality.
Similarities to Other Diseases and Stressors
The effects of bacterial leaf scorch are similar to those caused by oak wilt and Dutch elm disease, though there are some notable differences. Oak wilt and Dutch elm disease often kill their hosts within a few months. Bacterial leaf scorch cripples its hosts over a period of years. All three diseases promote a premature browning of infected leaves. This leaf browning is more uniform on plants affected by oak wilt or Dutch elm disease. The browning that results from bacterial leaf scorch infections starts along the leaf margins, and proceeds inwards toward the leaf veins.
Bacterial leaf scorch may be easily mistaken for drought or heat stress. Leaf scorch damage initially impacts older leaves, before spreading toward the branch tips. Environmental stressors tend to affect individual leaves, or entire branches.
- Bacterial leaf scorch has no known cure. As such, it is pivotal to manage infections in order to prevent diseased plants from rapidly declining.
- Branches that are dead or have declined due to bacterial leaf scorch should be excised from the plant. Sanitize pruning equipment between cuts using a solution composed of nine parts water and one part bleach. Always dry tools after disinfecting them.
- Severely weakened plants and plants that have lost over two thirds of their growth can be hazardous, and should be culled.
- When planting in locations where bacterial leaf scorch has been reported, select plant species that exhibit an increased resistance to the disease, such as ash, beech, or tulip poplar.
- Drought stress exacerbates infections. To limit infections, ensure that plants are sufficiently watered, especially during extended periods of drought.
- Apply a layer of organic mulch around the base of susceptible plants to improve the soil quality, moderate the soil temperature, and retain soil moisture.
- Annual treatments of an Oxytetracycline antibiotic are required to suppress the disease. The antibiotic can be administered via a root flare injection. While it will not cure the disease, the antibiotic will limit bacterial growth for several weeks, delaying infection symptoms.
- Rake and dispose of leaves shed from diseased plants.
Photo courtesy of R. A. Melanson, Mississippi State University